Compared with CRT (Cathode-Ray Tube) displays which have been widely used, liquid crystal display devices are flatter, lighter, consumes smaller energy, and are capable of having high definition. Due to such characteristics, liquid crystal display devices are widely used not only for portable apparatuses but also for monitors of laptop computers and desktop computers. However, liquid crystal display devices are inferior to CRT displays in that the liquid crystal display devices have a slower response speed and lower quality of moving pictures. For that reason, various methods have been discussed so as to improve liquid crystal display devices in terms of liquid crystal materials, panel structures, driving methods, and the like.
Patent Citation 1 (Japanese Patent No. 2650479; published on Jul. 29, 1991) discloses a driving method as described below. In a case where a gradation transition is not completed within a rewrite time (16.7 μm) corresponding to a frame frequency (60 HZ), a liquid crystal display device using the driving method carries out a gradation transition from a previous gradation to a current gradation so that a current driving signal is modulated, thereby completing a response in one frame. The following explains the method with reference to FIGS. 20 and 21.
As an example, in a liquid crystal panel having a TN (Twisted Nematic) liquid crystal in a reflective mode and having a minimum voltage of 2.0V at which a liquid crystal does not transmit light and having a maximum voltage of 3.5V at which the liquid crystal transmits a maximum amount of light, it is assumed that when an applied voltage V1 of 2.0V is applied until a frame FR(2) ends and the applied voltage V1 is changed to V5(2.5V) in and after a next frame FR(3), a transmittance amount of a pixel in the liquid crystal panel changes as illustrated in FIG. 20.
In this case, a period from a time when the applied voltage changes to V5 to a time when a transmittance amount of the pixel reaches a predetermined value and luminance of the pixel reaches a desired value (luminance corresponding to V5) is approximately 70 to 100 msec. In this case, a response time for the pixel to have a desired transmittance amount (luminance) is two frames or more, so that image smearing occurs in an image displayed on the liquid crystal panel. Note that, “image smearing” in an image is a phenomenon in which transmittance of a liquid crystal does not change in line with a change in a voltage applied on a pixel and therefore a change in a display pixel causes an image of a previous field to be displayed shadowily at an outline of a current image. The phenomenon occurs when an image moves at a predetermined speed or more. The phenomenon greatly deteriorates image quality.
In general, a transmittance amount of a liquid crystal increases more rapidly as a larger voltage is applied. In a case where applying a voltage V5 in FR(3) would not allow luminance of a pixel to reach a desired value (luminance corresponding to V5) at a beginning of the next frame FR(4), voltage data is corrected so that a voltage higher than the voltage V5 is applied in the frame FR(3) where the voltage V5 is applied, thereby allowing for increasing a response speed of a liquid crystal. If the response speed of a liquid crystal display is more than a predetermined value, then it is possible to always complete a response of a liquid crystal within one frame.
To be more specific, a liquid crystal control circuit compares data of frame FR(2) and data of frame FR(3) so as to comprehend an amount of a voltage change in a pixel, and causes a data corrector (see FIG. 2 of Patent Citation 1) to correct the data of frame FR(3) from S5 to S7. Accordingly, a source driving IC (see FIG. 1 of Patent Citation 1) for driving a source signal line (data signal line) applies, on the source signal line, a voltage V7 corresponding to the corrected voltage data S7.
Therefore, rising characteristics of a liquid crystal are improved compared with a case where the voltage V5 corresponding to S5 which is not corrected is applied (a case of FIG. 20). Consequently, a desired transmittance amount T5 can be obtained in one frame which is FR(3). Note that, for convenience of explanation, in FIGS. 20 and 21, (i) a period during which data (e.g. S5) is supplied to a data corrector, (ii) a period during which the data corrector corrects the data and outputs generated data (e.g. S7), and (iii) a period during which a source driving IC applies a voltage (e.g. V7) corresponding to the corrected voltage data on a pixel are shown so that periods (i), (ii), and (iii) are disposed in a longitudinal direction, and the data or the voltage is referred to as data or a voltage of a frame (e.g. FR(3)). Further, a change in luminance of a pixel from a time when a voltage of one frame is applied to a time when a next voltage is applied is referred to as a change in luminance of the frame, and the change in luminance of the frame is shown so as to be disposed in a longitudinal direction under or above a period during which a voltage of the frame is applied.
As described above, a current driving signal is modulated by using a driving method disclosed in later-mentioned Citation 1, so that it is possible to always complete a response of a pixel in one frame if a response speed of a liquid crystal has a predetermined value or more.
However, in a case where a response of a liquid crystal is not completed in one frame although the above driving method is adopted, that is, in a case where a response of a liquid crystal is slow and a currently desired gradation is not realized even if a current driving signal is modulated so as to emphasis a gradation transition, a next driving signal is modulated and a next gradation transition is emphasized assuming that a current gradation transition has been completed in a transition from a current gradation to a next gradation. Consequently, next modulation may be performed incorrectly. Particularly in a change from decay to rise, a next gradation transition is emphasized too much, so that display quality may be greatly deteriorated. The following explains such a situation with reference to FIGS. 22 and 23.
FIG. 22 illustrates an example of changes in data, voltages, and a transmittance amount in a case where a gradation transition is emphasized. Here, a range of a driving voltage for a driving driver of a liquid crystal display element is limited. Furthermore, due to liquid crystal characteristics, a voltage whose r.m.s. value is 0V or less cannot be applied. For that reason, in a case of a low temperature at which response characteristics of a liquid crystal display element itself are lower than those at a normal temperature, or in a case where a response speed of a liquid crystal display element itself is slow, voltage application for emphasizing a gradation transition cannot be performed, so that a response of a liquid crystal may not be completed in one frame.
FIG. 22 illustrates a case where input data changes from S5 to S1 in a gradation transition from frame FR(2) to frame FR(3). In this example, a change in a transmittance amount lasts three frames, that is, a response time to reach a desired transmittance amount requires three frames.
Under the circumstance, assume that data S5 is supplied in FR(4). At that time, data changes from S1 to S5. Therefore, if a gradation transition is emphasized so that data changes from S1 to S7 and a driving voltage V7 corresponding to S7 is applied as with the case of FIG. 21 in which a pixel has already reached a transmittance amount corresponding to S1, then the gradation transition is emphasized too much.
To be specific, assume that, as illustrated in FIG. 23, a gradation transition is emphasized so that data changes from S1 to S7 as with the case of FIG. 21, although a response of a transmittance amount from S5 to S1 is not completed in one frame. At that time, at the end of frame FR(3), although transmittance amount T1 corresponding to data S1 is not yet realized, a voltage V7 is applied so that a transmittance amount changes from T1 to T5. Consequently, the gradation transition is emphasized too much. As a result, a transmittance amount of a pixel at the end of frame FR(4) exceeds a desired transmittance amount T5. At that time, a user recognizes excess brightness on a display device. This results in great deterioration in display quality.
On the other hand, Patent Citation 2 (Japanese Patent No. 2708746; published on Jan. 13, 1989) discloses an arrangement in which: instead of storing gradation data of a current frame in a frame memory till a next frame begins, data determined by estimating a state of a liquid crystal at the beginning of a next frame is stored in the frame memory.
To be specific, a correction circuit estimates that if a voltage corresponding to gradation data supplied in a current frame is applied on a liquid crystal, then what gradation corresponds to transmittance of the liquid crystal after one frame, and the correction circuit writes data indicative of the gradation in the frame memory and causes the frame memory to store the data till a next frame begins.
As a result, data read from the frame memory in each frame is data indicating that if a voltage corresponding to gradation data supplied in a previous frame is applied on a liquid crystal, then what gradation corresponds to transmittance of the liquid crystal in a current frame which is one frame after the previous frame. Therefore, unlike an arrangement in which gradation data of a previous frame is stored till a next frame and the gradation data of the previous frame is compared with gradation data of a current frame so as to correct the gradation data of the current frame, if estimation is correct, too much correction can be prevented, so that excess brightness can be prevented.
In the arrangement, if estimation is correct, then it is possible to prevent deterioration in image quality due to too much correction. However, if estimation has errors, then the errors are accumulated and it may be difficult to perform suitable correction.
Consequently, accuracy in the estimation must be maintained so that accumulation of errors does not result in great deterioration in image quality. This increases an amount of calculation for estimation and a size of a circuit necessary for the estimation.